The hypothalamic hormone GnRH (sometimes referred to as LH-RH or as LRF) which triggers the release of the gonadotropic hormones, particularly LH and FSH, was isolated over 15 years ago and characterized as a decapeptide having the structure: EQU pGlu--His--Trp--Ser--Tyr--Gly--Leu--Arg--Pro--Gly--NH.sub.2
Peptides are compounds which contain two or more amino acids in which the carboxyl group of one acid is linked to the amino group of the other acid. The formula for GnRH, as represented above, is in accordance with conventional representation of peptides where the amino terminus appears to the left and the carboxyl terminus to the right. The position of an amino acid residue is identified by numbering the amino acid residues from left to right. In the case of GnRH, the hydroxyl portion of the carboxyl group of glycine at the C-terminus has been replaced with an amino group(NH.sub.2) i.e. the C-terminus is amidated. The abbreviations for the individual amino acid residues above are conventional and are based on the trivial name of the amino acid, e.g. pGlu is pyroglutamic acid, Glu is glutamic acid, His is histidine, Trp is tryptophan, Ser is serine, Tyr is tyrosine, Gly is glycine, Leu is leucine, Nle is norleucine, Orn is ornithine, Arg is arginine, Har is homoarginine, Pro is proline, Sar is sarcosine, Phe is phenylalanine, Ala is alanine, Val is valine, Nva is norvaline, Ile is isoleucine, Thr is threonine, Lys is lysine, Hly is homolysine, Asp is aspartic acid, Asn is asparagine, Gln is glutamine, and Met is methionine. Except for glycine, amino acids of the peptides described herein should be understood to be of the L-configuration unless noted otherwise.
There are reasons for desiring to prevent ovulation in female mammalians, and the administration of GnRH analogs that are antagonistic to the normal function of GnRH have been used to suppress or delay ovulation. For this reason, analogs of GnRH which are antagonistic to GnRH are being investigated for their potential use as contraceptives or for regulating conception periods. GnRH antagonists may also be used for the treatment of precocious puberty and endometriosis. Such antagonists have also been found useful to regulate the secretion of gonadotropins in male mammals and can be employed to arrest spermatogenesis, e.g. as male contraceptives for treatment of male sex offenders, and for treatment of prostatic hypertrophy. More specifically, GnRH antagonists can be used to treat steroid-dependent tumors, such as prostatic and mammary tumors. In the female, they can also be used for hirsutism.
A number of analogs of GnRH which have been synthesized showed high biological potency in inhibiting the secretion of the gonadotropins, such as those described and claimed in U.S. Pat. No. 4,444,759 issued Apr. 24, 1984 to Rivier and Vale. However, subsequent testing of such GnRH antagonists found that they also exhibited the undesirable side effect of causing the release of relatively large amounts of histamine, which could cause edema of the face and extremities. These overall effects of such synthetic analogs were reviewed in an article by Karten and Rivier which appeared in Endocrine Reviews, 7, pp. 44-66 (Feb. 1986).
Because it was felt that such side effects would likely prevent administering such antagonists to humans, other GnRH antagonist designs were sought. Cyclic GnRH antagonists are shown in U.S. Pat. No. 4,661,472 issued Apr. 28, 1987, and GnRH antagonists having the side chain of D-Glu or a similar residue in the 6-position modified (for example to form D-4-p-methoxybenzoyl-2-amino butyric acid) are shown in U.S. Pat. No. 4,677,193, issued Jun. 30, 1987.
Another proposal of investigators was to modify the D-Arg residue in the 6-position by substituting its guanadino side chain with alkyl groups, as shown in the article by Nestor, et al. Peptides 88, pp. 592-594. In an article by Rivier, et al., entitled GnRH Antagonists: N-Alkylation of Primary Amino Functions Generate New Potent Analogs, Coll. Soc. Fr. Etudes Fertil. (1988) 26, pp. 25-31, there is described the design and synthesis of potent GnRH antagonists which use N-alkylated lysine residues in the 6- and/or 8-positions.
Published International Application W089/01944 discloses a series of decapeptide analogs of GnRH which have antagonistic properties and which release only low amounts of histamine; one of these compounds, termed Antide, utilizes NicLys in the 5-position and D-NicLys in the 6-position. By Nic is meant the acyl group nicotinoyl or 3-pyridylcarbonyl. Such GnRH antagonists appear to have commercial possibilities; however, the synthesis of these compounds is timestaking and relatively expensive. p-Nitrophenol nicotinate is first prepared by the reaction of nicotinic acid with nitrophenyl; it must then be precipitated, recrystallized and checked for purity and identity. This active ester is then reacted with lysine protected by Boc using a reaction that takes 36 hours, the product must again be recovered, recrystallized and checked for identity and purity. The resultant compound is then utilized as a part of a step-by-step solid phase synthesis of the decapeptide as is well known in the art. In order to make such decapeptides more readily available, it was felt that more efficient syntheses would be highly desirable.